Earlier quoted context omitted.
Loans that are likely to default have higher interest rates. However, jumbo loans also have high interest rates due to the size of the principal. Although they're supposed to be less likely to default, being held by high-income individuals, if they do default then you lose more principal than a regular loan. For a bank in good times, you get to charge high interest rates normally reserved for risky borrowers on safe…
But in a well working market, why wouldn't bank X provide a lower rate than bank Y if the loan is less risk and very profitable? And then bank Z sees a potential profit and undercuts bank X. Repeat until you get a rate that reflects the risk? So the real question is: What is wrong in the high-end US mortgage market? Are banks breaking anti-trust laws and setting price floors?
Risk 1: High chance of default. This is the risk you take by lending to borrowers with poor credit.
Risk 2: Low chance of default, but when defaults happen you take a big hit. These are jumbo loans.
Banks recognize that Risk 2 has inherent costs that require a higher interest rate. My previous wording of "More profit with less risk" was false. More profit with less Risk 1, but that's because you're taking on Risk 2.